class_docstr: Base class for Air's Keras callbacks.
code: def _handle(self, logs: Dict, when: str):
      raise NotImplementedError
  
imports: import shutil
  from typing import Dict, List, Optional, Union
  from tensorflow.keras.callbacks import Callback
  import ray
  from ray.train.tensorflow import TensorflowCheckpoint
  from ray.util.annotations import PublicAPI
label_desc: Use image node_id 2 for calling the _Callback obj's underlying member method code with example usage: obj._handle(logs, when) without return types